The present invention relates to a novel process for the extraction and recovery of vanadium and molybdenum, as well as cobalt and/or nickel, from a spent desulfurization catalyst.
Desulfurization catalysts are extensively used for the catalytic hydro-desulfurization of petroleum fractions having a sulfur content sufficiently high to produce atmospheric pollution when such fractions are burned as fuels. Treatment of the petroleum fraction with hydrogen at suitable temperatures and pressures in the presence of the catalyst results in the conversion of the sulfur to hydrogen sulfide, which may be recovered. As a result of this treatment, the desulfurized fractions can be used as clean fuels, which produce little or no air pollution.
The conventional desulfurization catalysts, in their freshly prepared state, comprise an aluminum oxide carrier upon which there are deposited compounds of molybdenum and cobalt as the active ingredients. In the course of the desulfurization some of the hydrogen sulfide formed reacts with the catalyst components to form sulfur-containing metal compounds, such as metal sulfides. During desulfurization, moreover, the catalyst gradually takes up vanadium and nickel from the petroleum until these and other impurities have so reduced the activity of the catalyst that insufficient desulfurization takes place and the spent catalyst must be replaced with fresh catalyst.
The spent catalyst therefore may contain in vanadium, molybdenum, cobalt and nickel, as well as aluminum oxide. These metals may be recovered for re-use, either in the preparation of fresh catalyst, or for other industrial purposes. Various methods have been proposed in the prior art for the extraction and recovery of the catalyst components.
In Japanese Patent Specification 47-31892 there is disclosed a two-stage process in which the spent catalyst is first oxidatively roasted and the resulting product, which contains the oxides of molybdenum, vanadium, cobalt and nickel, is mixed with an aqueous solution of sodium hydroxide, sodium carbonate, sodium bicarbonate, and the like, after which the mixture is calcined at a temperature above 600.degree. C. These alkaline decomposition agents are used in an amount which is 1.2 to 1.5 times the theoretical amount needed to convert vanadium and molybdenum into water-soluble salts, such as sodium vanadate NaVO.sub.3, and sodium molybdate Na.sub.2 MoO.sub.4. The greatest disadvantages of this method are the cumbersome two-stage procedure, and the problem of roasting gas disposal (emission of sulfur dioxide) presented by the first stage. By using an oxidative roasting stage in treating such a spent desulfurization catalyst, however, the environmental advantages obtained by the desulfurization of the petroleum fractions are partly undone unless costly provisions are made for controlling undesirable sulfur dioxide emissions.